This invention relates to an optical fiber which has excellent transmission characteristics and can be used ideally in an optical fiber cable and an optical fiber coil.
An optical fiber has inside it a glass part made up of a core and a cladding, and has outside this cladding a resin covering layer made up of one layer or a plurality of layers. It is well known that when an optical fiber suffers a side pressure it undergoes local bending, and this bending results in a worsening of transmission loss. Transmission loss caused by bending is called xe2x80x98bend lossxe2x80x99. There are two different types of bend loss: xe2x80x98micro bend lossxe2x80x99, which occurs at local minute bend parts resulting for example from adjacent optical fibers overlapping and exerting side pressures on each other; and xe2x80x98macro bend lossxe2x80x99, which occurs as a result of an optical fiber itself describing an arc when it is laid. If the radius of curvature of the arc described by an optical fiber itself is large, macro bend loss does not occur (or is small enough to be ignored).
Because bend loss occurs at various locations on an optical transmission path, even if the loss at any one location is not that large, over the whole optical transmission path it may become too large to be ignored. Consequently, optical fibers which are resistant to bend loss are being sought after. And also in the case of optical fiber cables having optical fibers inside them, cables which are resistant to bend loss are being sought after.
Also, optical fiber coils, which are formed by winding an optical fiber into a coil, tend to have increased bend loss. An optical fiber coil is an optical component used in optical amplifiers and wavelength-dependent dispersion compensators, polarization mode dispersion compensators and optical fiber gyros and the like. In optical fiber coils of this kind there is a tendency for the above-mentioned resin covering layer to be made thin in order to make the optical fiber in the coiled state compact, and in this case micro bending is more liable to occur. Consequently, in optical fiber coils there is a tendency for worsening of transmission loss caused by bend loss to occur readily.
A known optical fiber coil is disclosed for example in Japanese Patent Application Laid-open No. H10-123342. And specific products include for example Sumitomo Electric Co., Ltd. coils of type numbers DCFM-340, DCFM-680, DCFM-1020 and DCFM-1360. An optical fiber coil exerts a desired effect on an optical signal along its optical path. For example, an optical fiber coil used in an optical amplifier is an optical fiber doped with erbium (Erbium Doped optical Fiber, or EDF) made into a coil, and amplifies an optical signal along the optical path of the optical fiber.
Here, because to amplify light an EDF of a certain length is necessary, the optical fiber is coiled so that it can be received efficiently inside the optical amplifier. The same applies to optical fiber coils used in other optical components besides optical amplifiers, such as wavelength-dependent dispersion compensators, polarization mode dispersion compensators and optical fiber gyros. Optical fiber coils have generally been made by winding an optical fiber onto a bobbin.
However, in a wound optical fiber there remains tension, and this causes micro bend loss to occur. And also, because stress resulting from deformation of the bobbin acts on the optical fiber due to a difference in linear expansion coefficient between the bobbin and the optical fiber, the transmission loss varies with temperature. To overcome this, as in the optical fiber coil disclosed in the above-mentioned application, various contrivances have been applied, and bobbin less optical fiber coils and bobbin structures with which the same effects as these can be obtained have been studied.
However, even with these various contrivances, it has not been possible to completely remove bend loss (micro bend loss) occurring as a result of minute bends in the optical fiber. And thus for optical fiber coils also, improvements providing further enhancement of transmission characteristics have been sought after.
An optical fiber provided by the present invention comprises a glass part having a core and at least one cladding and at least one covering layer formed around the glass part, and the Young""s modulus at 23xc2x0 C. of the covering layer, after the glass part is removed from it, is not greater than 400 MPa. The optical fiber referred to here is not only a single core optical fiber and also includes multi-core optical fibers (for example tape-form optical fibers).
Here, preferably, the covering layer is made two layers, an inner layer covering and an outer layer covering, and the Young""s modulus measured by the PIM method of this inner layer covering is made not greater than 0.8 MPa. If this is done, transmission loss can be reduced by an effect of the inner layer covering absorbing side pressures acting on the outer layer covering so that they do not reach the glass part.
And, here, the sliding friction coefficient of the outermost layer of the covering layer is preferably made not greater than 0.20. This sliding friction coefficient is a sliding friction coefficient relating to a covering layer of an optical fiber having inside it a glass part. The method by which the sliding friction coefficient is measured will be discussed later.
When this is done, contacting sites on adjacent optical fibers slide over each other readily; side pressure components other than components normal to the surfaces of the optical fibers are lightened; and the side pressures that the optical fibers exert on each other decrease. As a result, with an optical fiber according to this invention, transmission loss can be suppressed.
Also, an optical fiber cable having the optical fiber described above inside it is desirable. The optical fiber cable referred to here is an optical fiber cable having inside it an optical fiber and having outside that a protective covering sheath or the like. Inside the optical fiber cable, besides the optical fiber, there may also be provided a steel wire (high-tensile wire) for taking tensile forces acting on the optical fiber cable, and a spacer having a groove for receiving the optical fiber. The optical fiber cable referred to here includes both cables having inside them a plurality of optical fibers and cables having only one optical fiber, and each optical fiber may be single-core or multi-core.
Preferably, each optical fiber inside the optical fiber cable is a dispersion-compensating optical fiber. The dispersion-compensating optical fiber (hereinafter also called a DCF) referred to here is an optical fiber having a wavelength-dependent dispersion characteristic of the opposite sign to an optical fiber for transmission path use such as a single mode optical fiber, and is capable of canceling out wavelength-dependent dispersion in an optical transmission path. Normally, it is modulized and used as a DCFM (Dispersion-Compensating optical Fiber Module).
DCFMs are used in dispersion compensators and the like. A dispersion compensator is a device for canceling out wavelength-dependent dispersion in the 1.55 xcexcm wavelength band in order to carry out long-distance, high-capacity optical transmission in the 1.55 xcexcm band using a single mode optical fiber having its zero dispersion wavelength in the 1.3 xcexcm band. This kind of dispersion compensator is made by making into a coil for example by winding on a small-diameter bobbin a long DCF having a wavelength-dependent dispersion in the 1.55 xcexcm band of the opposite sign to that of a single mode optical fiber having its zero dispersion wavelength in the 1.3 xcexcm band.
Also, an optical fiber coil made by coiling the optical fiber described above is desirable. And preferably, in this optical fiber coil, the optical fiber is a dispersion-compensating optical fiber. An optical fiber coil using a dispersion-compensating optical fiber is as discussed above.
And in this kind of optical fiber coil, preferably, the coiled optical fiber is surrounded with a filler and the filler is a resin whose undisturbed penetration as prescribed in JIS K 2220 is in the range of 5 to 200 over the entire measurement temperature range of xe2x88x9240xc2x0 C. to 100xc2x0 C. and packs the whole coiled optical fiber and holds it in the coiled state. Undisturbed penetration is prescribed in Japanese Industrial Standards JIS K 2220-1993 (JIS K 2220-1993 2. (14), 5.3.1(4), 5.3.6 etc.). However, whereas in JIS K 2220 the measurement temperature is 25xc2x0 C., here, a resin whose undisturbed penetration is in the above-mentioned range over the entire measurement temperature range of xe2x88x9240xc2x0 C. to 100xc2x0 C. is used.